Originally Posted by NOSSLP
Take off the belt, and use some 320 grit sandpaper on all of the pulleys, then put on another new belt. If this doesn't help, you may have a bad idler pulley, which are easy to change.
x2 - 400 grit also works, same does steal wool (i think thats what you call it). Get the pulleys real good and then clean them with brake cleaner. Do the same to the belt (easier on the belt tho, remember it is rubber) but AS SOON as you get the money I would replace the belt with either a Gates or a Gatorback. Probubly be around $40-$50 for the belt but its worth it for peice of mind and acually driving without the radio cranked to drown the squeak... lol.
What happens is the belt (from slipping) and pulleys get "glazed", you can tell that they are glazed by looking at the pulley (not the ribbed ones the flat ones) and if it appears shiney or like a mirror it is glazed. Same for the belt, if its really smooth and shiney its glazed. The sandpaper removes that glaze and "scratches" the surface (for lack of a better word) which in return causes the belt to grip better and less squeak.